Recently, demands for solid-state image sensors (to be referred to as image sensors hereinafter) are increasing as digital cameras, cell phones with cameras, and the like have become popular.
An image sensor mainly includes a pixel region and peripheral region. Photoelectric conversion elements (e.g., photodiodes) for converting incident light into electrical signals are formed in the pixel region. Circuits for processing the signals from the pixel region and circuits for controlling the operation of the pixel region are formed in the peripheral region. These circuits are fabricated by using MOS (Metal-Oxide-Semiconductor) transistors and the like. The photodiodes and MOS transistors are formed on, e.g., the same semiconductor substrate.
In a front-illuminated image sensor, light enters from a surface on which the gate electrodes of the MOS transistors and interconnects are formed. This front-illuminated image sensor has the problem that as the degree of downsizing of the pixels (photoelectric conversion elements) increases, the sensitivity to light entering the pixel region decreases owing to the reduction in area of the pixel region or light shielding caused by the interconnects connected to the circuits.
To solve the problem of the front-illuminated image sensor as described above, a back-illuminated image sensor has been proposed. The back-illuminated image sensor receives incident light from a surface opposite to the surface on which the gate electrodes of the MOS transistors and the interconnects are formed. This avoids the decrease in sensitivity caused by the downsizing of the pixels and the light shielding by the interconnects.
In a conventional back-illuminated image sensor manufacturing method, after elements and interconnects are formed on a semiconductor substrate, a support substrate is adhered on an insulating film covering the elements and interconnects. After that, lenses and color filters are attached to a surface of the substrate, which is opposite to the support substrate.
In general, when using aluminum (Al) as interconnects, after the Al interconnects are formed on a lower interconnecting layer, an insulating film is deposited on the Al interconnects and between adjacent Al interconnects. In this case, even when the deposited insulating film undergoes a planarizing process, the upper surface of the insulating film cannot have high flatness owing to, e.g., junction portions (seams) of the insulating film. Consequently, when adhering the insulating film and support substrate of the back-illuminated image sensor, no high adhesion can be obtained between the insulating film and support substrate, and the support substrate peels off. This poses the problem that adhesion defects occur between the insulating film and support substrate.